docs: def write_libraries(dir, libraries):
    """Write a list of libraries to disk.
  
    Args:
      dir: Output directory.
      libraries: List of (filename, library) pairs.
    """
    files = [open(os.path.join(dir, k), "w") for k, _ in libraries]
    # Document mentioned symbols for all libraries
    for f, (_, v) in zip(files, libraries):
      v.write_markdown_to_file(f)
    # Document symbols that no library mentioned.  We do this after writing
    # out all libraries so that earlier libraries know what later libraries
    # documented.
    for f, (_, v) in zip(files, libraries):
      v.write_other_members(f)
      f.close()